In the manufacture of printed circuit cards and boards a dielectric sheet material is employed as the substrate. A conductive circuit pattern is provided on one or both of the major surfaces of the substrate.
A conductive pattern can be formed on the surface of a substrate using a variety of known techniques. These known techniques include the subtractive technique where a layer of copper is etched to form the desired circuit pattern, the EDB (electroless direct bond) technique where copper is electrolessly plated directly on the surface of the substrate in the desired pattern, the peel-apart technique where the desired circuit pattern is plated up from a thin layer of peel-apart copper.
In addition, connections between layers are made by means of plated through-holes or vias. In plating such holes, copper must be plated directly on the dielectric substrate (on the walls of the holes or vias). In addition, if one employs the EDB technique, it is necessary to plate directly on the surface of the substrate.
Since the dielectric substrate is non-conductive, in order to plate on the substrate (either on the walls of the holes or on the surface of the substrate) the substrate must be catalyzed prior to the deposition of metal onto the substrate in order to render it susceptible to being plated by an electroless plating technique.
Among the more widely employed procedures for catalyzing a substrate is the use of a stannous chloride sensitizing solution and a palladium chloride activator to form a layer of metallic palladium particles thereon.
For instance, one method for catalyzing a dielectric substrate is exemplified by U.S. Patent 3,011,920 which includes sensitizing the substrate by first treating it with a solution of a colloidal metal, accelerating the treatment with a selective solvent to remove protective colloids from the sensitized dielectric substrate, and then electrolessly depositing a metal coating on the sensitized substrate; for example, with copper from a solution of a copper salt and a reducing agent.
Also, as suggested in U.S. Pat. No. 3,099,608, a dielectric substrate can be pretreated by depositing a thin film of a "conductivator" type of metal particle, such as palladium metal, from a semicolloidal solution onto the dielectric substrate to provide a conducting base that permits electroplating with conductive metal on the conductivated base.
U.S. Pat. No. 4,066,809 discloses the use of a so-called "triple seeding" technique which includes contacting the surface of the dielectric material with an aqueous stannous chloride sensitizing solution followed by contacting the surfaces of the dielectric material with an aqueous palladium chloride activator solution and then followed by contacting the surfaces of the dielectric material with an aqueous palladium chloride/stannous chloride/hydrochloric acid seeder bath.
As can be appreciated, although the prior art methods of sensitizing or conditioning a dielectric substrate for subsequent plating from an electroless metal plating bath are adequate for the most part, they require a multi-stepped process (e.g., sensitizing the substrate and then activation of the sensitizing materials, such as the stannous compound, by reaction with a palladium compound or the like).
Moreover, the prior art methods have been somewhat limited in the particular combination of metals that provide adequate results. The prior art has also been somewhat restrictive in the actual form by which the metallic materials could be successfully applied to the desired substrate.